One Needs Power and a Place to Drain; the Other Goes in the Box
A dehumidifier is a powered appliance that pulls moisture from the air of a room or space and collects it as water you empty or drain. A desiccant is a passive material that absorbs moisture inside a sealed package, crate or container with no power, no drain and no moving parts. They solve the same humidity problem at completely different scales and places, so they are rarely interchangeable.
If you are drying a warehouse, a workshop or a storage room, a dehumidifier is the tool. If you are protecting goods sealed inside packaging while they ship or sit in store — where there is no power and no one to empty a tank — a desiccant is the only practical option. That is the entire reason container and export protection runs on desiccants: a 30-day ocean voyage has nowhere to plug in a dehumidifier.